Gas turbine engines operate at extremely high temperatures for increased performance and efficiency. A limiting factor in most gas turbine engine designs, however, is the maximum temperature that various components of the engine can tolerate. One such particular component area which is so limited is the combustion chamber of a gas turbine engine.
One method to increase the maximum allowable temperature and/or decrease the component metal temperature is to provide cooling holes in the walls of the component. These holes allow cool air to flow through and along the walls of the component exposed to the high gas temperatures. As the air flows along the surface of the walls it forms a cool layer. This cool layer reduces the temperature of the wall surface and physically keeps the hot gases from contacting the walls of the component, thereby permitting the component to withstand higher gas temperatures than would otherwise be possible.
Another method of allowing higher gas temperatures to be used is to apply a protective thermal barrier coating to the walls of the component that are exposed to the hot gases. In the case of combustors this is, in particular, the inner walls of the flame tube, the outer walls being exposed to cooler compressor delivery air. Such coatings conventionally comprise, for example a MCrAlY material which offer thermal and corrosion protection. MCrAlY refers to known coating systems in which M denotes nickel, cobalt,iron or mixtures thereof; Cr denotes chromium; Al denotes aluminium; and Y denotes yttrium. A further ceramic layer is also often applied on top of the MCrAlY layer to give improved thermal protection. In such an arrangement the MCrAlY layer acts as a bond coat for the ceramic coating layer. An example of such a ceramic coating material is yttria stabilised zirconia which is applied on top of an MCrAlY layer.
The MCrAlY and ceramic protective coatings are typically applied by physical vapour deposition (PVD), chemical vapour deposition (CVD) or plasma spraying means. Examples of such protective coatings and the methods of applying them are well known and are described in: U.S. Pat. No. 4,321,311, U.S. Pat. No. 5,514,482, U.S Pat. No. 4,248,940 among many others.
Cooling holes and protective coatings can, and are, used in conjunction to allow operation of a component at a high temperature. There are two basic methods for producing such components that have cooling holes and a protective coating. In the first method the coating is applied to the component and then the holes are drilled through the coated component. Examples of this method are described in pending European Patent Application Number 97305454 in which laser drilling is used to penetrate a thermal barrier coating and the metal of the component. A problem with this method is that, by design, the thermal barrier coating is resistant to heating produced by the laser to drill through the material. Consequently drilling of the coating requires a high power laser, a prolonged operation, and results in considerable heating of the surrounding area which can be undesirable. Problems also exist if mechanical drilling techniques are used since the thermal barrier coatings are generally brittle. Mechanical drilling can crack and damage the coating in the region around the holes causing the coating to fall off the component either during the machining operation or prematurely during service.
In the second method holes are drilled in the component and then the coating is applied to the drilled component. This method does not have any of the problems associated with drilling/machining through the coating described above. However application of the coating after the holes have been drilled does tend to at least partially block some or all of the holes. This restricts the flow of cooling air through the holes and can result inadequate cooling of the component producing hot spots, overheating and possible failure of the component. Furthermore the blocking of the cooling holes is unpredictable and so designing the holes to accommodate a degree of blockage is problematic and also, if it is possible will reduce the efficiency of the engine.
Consequently any coating material blocking the cooling holes has to be removed. The problem of cooling hole blockage and a method of removing the coating from a cooling hole is described in EP 0,761,386. According to this patent an abrasive slurry under pressure is directed at the coating on the component. This slurry flows through the cooling holes thereby removing the coating material that is blocking the hole. A similar technique using a high pressure fluid jet is also described in JP 8108400.
A problem with both of these methods of clearing the holes is that the high pressure fluid jet, and the abrasive slurry, as well as removing the coating material from within the hole can also undesirably damage the remainder of the coating on the component. In particular the coating material in the region around the cooling hole is often damaged. This damage to the coating can reduce the coating thickness and/or reduce its adhesion to the component resulting in the coating falling off.
Further problems are that the high pressure fluid, and the abrasive slurry, have to be accurately directed at the specific cooling holes. This requires that the high pressure fluid jet, or abrasive slurry, be accurately controlled and directed. This is however difficult to achieve in a production environment and the machines capable of such accurate control are expensive. An alternative way of accurately directing the abrasive slurry or jet at a particular hole is to use a protective maskant or suitable tooling to mask some of the holes, and possibly the surrounding area of the coating. The mask protects the areas of the component from the jet or abrasive slurry and accurately directs the high pressure fluid or abrasive slurry into the holes. The mask is then removed and applied to a different area and cooling holes in order to clear all of the holes of the component. This process is, however, slow and is not conducive to a production environment. There is also a possibility that if a maskant is used that the maskant may not be fully removed and may itself block the cooling holes. A maskant, or the method of removal of the maskant, may also damage the coating.